The invention relates to a method for controlling the current of a voltage controlled oscillator, and respectively, to a circuit arrangement for controlling the current of a voltage controlled oscillator.
One of the most important objectives given to a designer of modern mobile phones, particularly the small-sized so called pocket phones, is to maximize the operating time. Thus the minimization of the operating current of the device is one of the most important design criteria. At the same time the goal is to provide as light and small-sized devices as possible, and thus also the number of battery cells generating the operating current should be limited. In other words, the objective should be an operating voltage which is as low as possible.
FIG. 1 shows the basic configuration of a typical prior art voltage controlled oscillator (VCO) in which a target was to observe the above mentioned requirements concerning the operating current and voltage. Here the VCO comprises an actual oscillating circuit or oscillator and an isolating amplifier, which is required so that changes in the environment do not substantially affect the operation of the oscillator. In order to minimize the power consumption the oscillating circuit and the amplifier circuit in this embodiment are "superimposed", in other words, the same supply current, a DC current, flows through both stages, as is seen in FIG. 1.
The oscillating circuit is formed by the transistor Q2 with associated passive components. The isolating amplifier circuit is formed by the transistor Q1 with associated passive components. The transistors Q1 and Q2 are "superimposed", or the same current flows through both transistor stages Q1 and Q2. A disadvantage of this configuration is that the total operating voltage increases, because it is formed by the sum of the operating voltages of both stages. The DC signal controlling the resonance frequency of the oscillating circuit is supplied to the interface Vctrl.
The combined operating voltage B of the circuits, which in FIG. 1 is typically 4.5 V, is suitably divided between the transistors. When the resistor R5 between the emitter of the transistor Q2 and ground is suitably selected, the current flowing through the transistors Q1 and Q2 will remain almost constant regardless of changing conditions, primarily temperature. However, a disadvantage of this configuration is that in spite of the constant current the output voltage may vary substantially, primarily due to parameter variations between individual transistors and tolerances of the passive components of the configuration.
The superimposed transistor stages of FIG. 1 inevitably result in an increased total voltage requirement, compared to transistor stages supplied directly from the voltage supply, because the operating voltage B is now the sum of the voltages applied over the transistor stages (for instance B=4.5 V). In the arrangement of FIG. 1 a considerable part of the operating voltage is "lost" in the emitter resistor R5 of the oscillator stage. In the case of FIG. 1 a suitable value of the resistor is e.g. 270 .OMEGA.. This resistance could be reduced, but in practice one can take about 100 .OMEGA. as the lower limit, below which the stability of the bias voltage (DC bias) of the transistor stage is lost. This is a well known situation to a person skilled in the art, so there is no need to describe the basic circuit theory in more detail. Also at the lower resistance value of 100 .OMEGA. the emitter DC bias will remain at a level of about 1 V, when a value of about 10 mA is used as the operating current through the stages in the arrangement. The result of this is, that even at the lowest level the total operating voltage generated by the battery B should be about 3 V.
The U.S. Pat. No. 4,968,952 presents a VCO with automatic current control. This patent aims at a control circuit which detects a direct current (DC) signal representing the oscillating current of the VCO circuit, in other words the oscillating state of the VCO circuit, and this signal is then used to control the VCO oscillator. As is presented in more detail in this patent description, a signal representing the voltage level is obtained either from the emitter or collector of the VCO oscillator's transistor, and then the alternating voltage (AC) component is intentionally filtered away in order to have a feedback signal comprising only the DC component. The object of the feedback signal is to keep a constant output power of the VCO so that also the oscillator noise is kept at a low level. However, the arrangement does not aim at minimizing the oscillator current, and the arrangement is not suitable to be used at low operating voltages due to the resistive components at both the emitter and collector of the transistor.